In modern helicopters, the trend toward main rotor systems which have lower inertia reduces the amount of stored energy in the rotor system and causes the rotor to be more susceptible to large transient speed excursions during certain flight maneuvers, e.g., during a sudden yaw maneuver changing the heading of the aircraft. Such main rotor speed excursions, working in conjunction with other flight characteristics of helicopters, may result in unbalanced torque causing the nose of the aircraft to deviate from the desired heading. This undesirable deviation in the aircraft heading may cause an increase in pilot workload, frequently at critical times, or saturation of the aircraft stability augmentation system, or both.
Typically, the helicopter main rotor and a tail rotor are driven by a common gear mechanism. The tail rotor primarily functions as a torque compensating device to counteract the torque effect of the main rotor. Secondarily, the tail rotor is a means for controlling the angular movement of the helicopter about its vertical axis, thereby controlling the heading of the helicopter. In newer generation helicopters, a shrouded fan rather than a conventional tail rotor may be used for torque compensation. As used herein, the term "tail rotor" is intended to refer to helicopter anti-torque devices including the conventional type of torque compensation device, i.e., a tail rotor, and the newer generation tail fan devices.
The thrust produced by the tail rotor is dependent upon the pitch angle of the tail rotor blades and, to a certain extent, upon external factors such as ambient wind conditions and main rotor down draft and RPM. Assuming that the main rotor rotates counterclockwise and the tail rotor is a puller type device, e.g., produces thrust to pull the tail in the direction of the thrust, when the tail rotor blades have a positive pitch angle, the tail rotor thrust tends to pull the tail to the right; if the blades have a negative pitch angle, the tail rotor thrust tends to the pull the tail to the left; and if the blades have a zero pitch, the tail rotor produces little or no thrust in either direction.
The pitch angle of the tail rotor blades is determined by the position of a pair of pilot operated anti-torque pedals. Typically, with the right pedal moved forward of a neutral position, the blades either have a negative pitch angle or a small positive pitch angle. The farther forward the right pedal is positioned, the larger the negative pitch angle, the nearer the right pedal is to the neutral position, the more positive the pitch angle. As the left pedal is moved forward of the neutral position, the positive pitch angle increases until it becomes maximum with full forward displacement of the left pedal.
When the positive pitch is increased past a threshold position for a given main rotor loading, tail rotor thrust exceeds the thrust needed to overcome torque effects of the main rotor, thereby causing the helicopter nose to yaw to the left. While this method of directional control is excellent, the power requirement of the tail rotor is taken from the total power available from the engine, thereby reducing the power available to the main rotor.
When the tail rotor blades are in a zero or negative pitch position, the tail rotor thrust is acting in the same direction as the main rotor torque, and the helicopter nose will yaw to the right. In this case, the thrust produced by the tail rotor during the right yaw maneuver is typically less than the thrust produced by the tail rotor to counteract the main rotor torque effect prior to the right yaw maneuver, and any excess power available from the engine will be provided to the main rotor.
During certain yaw maneuvers, undesirable rotor speed excursions may occur. For example, during a rapid left turn (for rotor systems which rotate counterclockwise), the tail rotor blades are rapidly positioned to a high positive pitch position, and the engine must supply additional power to the tail rotor to produce sufficient thrust to cause the aircraft to rotate to the left about the yaw axis. During such rapid left turns, the combined demand for power of the tail rotor and main rotor exceeds the engine's current output, and there is insufficient time for the engine to spool up to meet the increased power demand. The tail rotor increased power demand is taken from the main rotor system, thereby causing the main rotor to droop because of loss of momentum. In addition, because the tail rotor and main rotor are driven by a common gear mechanism, the tail rotor may also experience the effects of the insufficient power, thereby resulting in the nose of the aircraft oscillating with a decaying amplitude around the desired heading.
Similarly, during a rapid right turn, the tail rotor blades are rapidly positioned to a zero or negative pitch position, and the power demand of the tail rotor is suddenly reduced. Therefore, the combined power demand of the main rotor and tail rotor is less than the power currently being produced by the engine. The excess power is delivered to the main rotor system, which may cause the main rotor to overspeed.
The insufficient power to meet the rapid increase in power demand caused by a sudden left yaw input, and the excess power available during a sudden right yaw input, is the result of the sluggishness of the helicopter's automatic fuel compensation system, often coupled with the pilot trying to compensate manually for the situation himself. Hence, a reduction in the accuracy of controlling the heading of the aircraft will result. In aircraft used for military purposes, a further negative effect is the reduced accuracy of aiming weapons. An additional problem associated with the sluggishness of the automatic fuel compensation system is that during recovery from main rotor droop, the automatic fuel compensation system initially will add additional fuel to expedite recovery from rotor droop; however, the system may fail to subtract fuel in sufficient time to prevent the rotor speed from exceeding 100% rated rotor speed during such recovery, therefore resulting in overshoot in the recovery from rotor droop causing the rotor to overspeed.